Crystallization behavior of (GeS2)0.1(Sb2S3)0.9 glass

The crystal growth kinetics of antimony trisulfide in (GeS₂)_0.1(Sb₂S₃)_0.9 glass has been studied by microscopy and DSC. The linear crystal growth kinetics has been confirmed in the temperature range 492 ? T ? 515 K (E_G = 405 ± 7 kJ mol⁻¹). The applicability of standard growth models has been assessed. From the crystal growth rate corrected for viscosity plotted as a function of undercooling it has been found that the most probable mechanism is interface controlled 2D nucleated growth. The non-isothermal DSC data, corresponding to the bulk sample, can be described by the Johnson-Mehl-Avrami equation.     

Kinetic analysis of solid‐state reactions: Precision of the activation energy calculated by integral methods

The integral methods are extensively used for the kinetic analysis of solid‐state reactions. As the Arrhenius integral function [p(x)] does not have an exact analytical solution, different approximated equations have been proposed in the literature for performing the kinetic analysis of experimental integral data. Since the first approximation of Van Krevelen, a large number of equations have been proposed with the objective of increasing the precision in the determination of the Arrhenius integral, as checked from the standard deviation of the approximated function with regard to the real exact value of the integral. However, the main application of these equations is the determination of the kinetic parameters, in particular activation energies, and not the computation of the Arrhenius integral. A systematic analysis of the errors involved in the determination of the activation energy from these integral methods is still missing. A comparative study of the precision of the activation energy as a function of x and T computed from the different integral methods has been carried out. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 658–666, 2005     

Combined kinetic analysis for crystallization kinetics of non-crystalline solids

A new method of kinetic analysis that allows the combined analysis of data obtained under different experimental conditions, i.e. heating pathway, is presented for the crystallization of non-crystalline solids. The method consists of a two-stage procedure. Firstly, an optimization procedure is applied for the determination of the Johnson, Mehl and Avrami (JMA). This optimization procedure is based on the linearization of the rate equation throw a logarithmic transformation; where the correlation coefficient function is the objective function, and the JMA order is the adjustable parameter. The JMA order obtained from this optimization procedure allows one to determine in a second stage the other kinetic parameters, i.e. activation energy and pre-exponential Arrhenius factor. The method proposed here allows one to analyze in a combined way data obtained under any heating profile, because no assumptions are made on the heating profile for the derivation of the rate equation. The method is tested with computed and experimental curves obtained under different heating conditions. The crystallization kinetics of (GeS₂)_0.3(Sb₂S₃)_0.7 glass are studied from the combined analysis of several curves.     

Algorithm for normal random numbers

We propose a simple algorithm for generating normally distributed pseudorandom numbers. The algorithm simulates N molecules that exchange energy among themselves following a simple stochastic rule. We prove that the system is ergodic, and that a Maxwell-like distribution that may be used as a source of normally distributed random deviates follows in the N-->infinity limit. The algorithm passes various performance tests, including Monte Carlo simulation of a finite two-dimensional Ising model using Wolff's algorithm. It only requires four simple lines of computer code, and is approximately ten times faster than the Box-Muller algorithm.     

On the kinetic analysis of thermoanalytical diagrams obtained with the “quasi-isothermal” heating technique

A comparative study of “n-order”, Avrami—Erofeev and diffusion-controlled reactions has been carried out using thermoanalytical data, calculated by assuming a “quasi-isothermal” heating technique. The results reported support the hypothesis that a single diagram obtained with this technique allows the discernment of Avrami-Erofeev, “n-order” and diffusion-controlled reactions. This does not occur when a linear heating program is used, as was reported in previous papers. In addition, it is shown that a single thermoanalytical curve obtained with a “quasi-isothermal” heating program does not permit the determination of the actual value of n of those reactions following “n-order” kinetics. On the other hand, it is proved that the kinetic analysis of two thermoanalytical curves obtained with a linear heating program and a “quasi-isothermal” heating technique, respectively, would provide an excellent procedure for discerning the proper mechanism of solid-state reactions.     

Physicochemical properties of Cr/TiO2 solid solutions

Solid solutions of Cr³⁺ ions in TiO₂ have been prepared by impregnating commercial TiO₂ with CrO₃(aq) and annealing in O₂ at 770 K. X-Ray diffraction data show the absence of a segregated Cr₂O₃ phase. The magnetic and spectroscopic properties have also been investigated.

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Cr³⁺ TiO₂ TiO₂ CrO₃(.) O₂ 770 . Cr₂O₃. .

     

Status of the hard X‐ray microprobe beamline ID22 of the European Synchrotron Radiation Facility

The ESRF synchrotron beamline ID22, dedicated to hard X‐ray microanalysis and consisting of the combination of X‐ray fluorescence, X‐ray absorption spectroscopy, diffraction and 2D/3D X‐ray imaging techniques, is one of the most versatile instruments in hard X‐ray microscopy science. This paper describes the present beamline characteristics, recent technical developments, as well as a few scientific examples from recent years of the beamline operation. The upgrade plans to adapt the beamline to the growing needs of the user community are briefly discussed.     

Z4-Kerdock Codes, Orthogonal Spreads, and Extremal Euclidean Line-Sets

When m is odd, spreads in an orthogonal vector space of type⁺(2m + 2,2) are related to binary Kerdock codes and extremalline-sets in 2^{m + 1} with prescribed angles. Spreads in a 2m-dimensionalbinary symplectic vector space are related to Kerdock codesover Z₄ and extremal line-sets in with prescribed angles. These connections involve binary, realand complex geometry associated with extraspecial 2-groups.A geometric map from symplectic to orthogonal spreads is shownto induce the Gray map from a corresponding Z₄-Kerdock codeto its binary image. These geometric considerations lead tothe construction, for any odd composite m, of large numbersof Z₄-Kerdock codes. They also produce new Z₄-linear Kerdockand Preparata codes. 1991 Mathematics Subject Classification:primary 94B60; secondary 51M15, 20C99.     

Constant Rate Thermal Analysis (CRTA) as a Tool for the Synthesis of Materials with Controlled Texture and Structure

Some examples of the use of the CRTA method for the synthesis of materials with controlled texture and structure are given. BaTiO₃ has been obtained from the thermal decomposition of Barium Titanyl Oxalate (BTO) and Barium Titanyl Citrate (BTC) by controlling the reaction temperature in such a way that the partial pressure of the gases generated in the reaction was maintained constant at a value close to 10^-2 mbar. It has been shown that this method allows getting BaTiO₃ with crystal sizes considerably lower than those obtained by decomposing the same precursors by conventional methods. This small crystal sizes lead to the stabilisation of the metastable cubic phase with regards to the tetragonal phase. It has been also shown that the control of the CO generated in the carbothermal reduction of silica allows tailoring the phase composition of the silicon nitride obtained as final product. This revised version was published online in August 2006 with corrections to the Cover Date.